Based on recent NIH-supported human studies from our group and work from others, vitamin D has emerged as a potential modifier of type 2 diabetes (t2DM) risk. In longitudinal observational studies, higher vitamin D status is associated with up to 83% reduction in the risk of t2DM. However, the evidence to support general supplementation does not currently exist because the favorable association between vitamin D status and t2DM may be confounded by a variety of factors and there are no published trials specifically designed to test the effects of vitamin D on reducing t2DM risk; therefore, definitive conclusions cannot be drawn on the role of vitamin D for prevention of t2DM. The goal of this application, which extends our previous work and complements our ongoing work in this area, is to take the necessary next step to address the issue of causality and to quantify the protective benefit of vitamin D in t2DM risk, if present. We propose a multi-center, randomized, double-masked, placebo-controlled trial to be conducted among several experienced clinical sites across the U.S. to evaluate whether vitamin D is an effective intervention for prevention of t2DM among high- risk individuals (vitamin D and type 2 Diabetes, D2D trial). The proposed D2D trial, which will be conducted under the U01 mechanism, will be a primary prevention trial of adults at risk for t2DM, to test whether raising plasma 25OHD concentration delays 'time-to-diabetes- progression. The need for such a trial has been recognized in the recent literature, as we describe in the grant application. The present application is for a U34 Implementation Planning grant to plan the proposed study. We expect the final product of the U34 grant to be the timely submission of an application (U01) to support the conduct of the proposed clinical trial. The proposed D2D trial has a high potential impact in the clinically important areas of vitamin D and t2DM prevention with extensive public health implications. We expect study results to define the role of improving vitamin D status in modifying t2DM risk. If the trial confirms a favorable benefit/harm ratio of raising 25OHD, then vitamin D supplementation will be integrated into conventional medical approaches to prevent t2DM and ameliorate personal and societal disease burden. Finally, the impact of the study is heightened by the assembled team of highly qualified investigators with complementary expertise and experience in running nutrition-based multicenter trials, many of whom have worked together previously, which maximizes the likelihood of trial conduct success.